Methods of making microporous film products have been known for some time. For example, U.S. Pat. No. 3,832,267, to Liu, teaches the melt-embossing of a polyolefin film containing a dispersed amorphous polymer phase prior to stretching or orientation to improve gas and moisture vapor transmission of the film. According to the Liu '267 patent, a film of crystalline polypropylene having a dispersed amorphous polypropylene phase is first embossed prior to biaxially drawing (stretching) to produce an oriented imperforate film having greater permeability. The dispersed amorphous phase serves to provide microvoids to enhance the permeability of the otherwise imperforate film to improve moisture vapor transmission (MVT). The embossed film is preferably embossed and drawn sequentially.
In 1976, Schwarz published a paper which described polymer blends and compositions to produce microporous substrates (Eckhard C. A. Schwartz (Biax-Fiberfilm), "New Fibrillated Film Structures, Manufacture and Uses", Pap. Synth. Conf. (TAPPI), 1976, pages 33-39). According to this paper, a film of two or more incompatible polymers, where one polymer forms a continuous phase and a second polymer forms a discontinuous phase, upon being stretched will phase separate thereby leading to voids in the polymer matrix and increasing the porosity of the film. The continuous film matrix of a crystallizable polymer may also be filled with inorganic filler such as clay, titanium dioxide, calcium carbonate, etc., to provide microporosity in the stretched polymeric substrate.
Many other patents and publications disclose the phenomenon of making microporous thermoplastic film products. For example, European patent 141592 discloses the use of a polyolefin, particularly ethylene vinyl acetate (EVA) containing a dispersed polystyrene phase which, when stretched, produces a voided film which improves the moisture vapor permeability of the film. This EP '592 patent also discloses the sequential steps of embossing the EVA film with thick and thin areas followed by stretching to first provide a film having voids which, when further stretched, produces a net-like product. U.S. Pat. Nos. 4,452,845 and 4,596,738 also disclose stretched thermoplastic films where the dispersed phase may be a polyethylene filled with calcium carbonate to provide the microvoids upon stretching. Later U.S. Pat. Nos. 4,777,073; 4,814,124; and 4,921,653 disclose the same processes described by the above-mentioned earlier publications involving the steps of first embossing a polyolefin film containing a filler and then stretching that film to provide a microporous product.
With reference to U.S. Pat. Nos. 4,705,812 and 4,705,813, microporous films have been produced from a blend of linear low density polyethylene (LLDPE) and low density polyethylene (LDPE) with barium sulfate as the inorganic filler having an average particle diameter of 0.1-7 microns. It is also known to modify blends of LLDPE and LDPE with a thermoplastic rubber such as Kraton. Other patents, such as U.S. Pat. No. 4,582,871, disclose the use of thermoplastic styrene block tripolymers in the production of microporous films with other incompatible polymers such as styrene. There are other general teachings in the art such as the disclosures in U.S. Pat. Nos. 4,472,328 and 4,921,652.
Relevant patents regarding extrusion lamination of unstretched nonwoven webs include U.S. Pat. Nos. 2,714,571; 3,058,868; 4,522,203; 4,614,679; 4,692,368; 4,753,840 and 5,035,941. The above '863 and '368 patents disclose stretching extruded polymeric films prior to laminating with unstretched nonwoven fibrous webs at pressure roller nips. The '203 and '941 patents are directed to co-extruding multiple polymeric films with unstretched nonwoven webs at pressure roller nips. The '840 patent discloses preforming nonwoven polymeric fiber materials prior to extrusion laminating with films to improve bonding between the nonwoven fibers and films. More specifically, the '840 patent discloses conventional embossing techniques to form densified and undensified areas in nonwoven base plies prior to extrusion lamination to improve bonding between nonwoven fibrous webs and films by means of the densified fiber areas. The '941 patent also teaches that unstretched nonwoven webs that are extrusion laminated to single ply polymeric films are susceptible to pinholes caused by fibers extending generally vertically from the plane of the fiber substrate and, accordingly, this patent discloses using multiple co-extruded film plies to prevent pinhole problems. Furthermore, methods for bonding loose nonwoven fibers to polymeric film are disclosed in U.S. Pat. Nos. 3,622,422; 4,379,197 and 4,725,473.
It has also been known to stretch nonwoven fibrous webs using intermeshing rollers to reduce basis weight and examples of patents in this area are U.S. Pat. Nos. 4,153,664 and 4,517,714. The '664 patent discloses a method of incremental cross direction (CD) or machine direction (MD) stretching nonwoven fibrous webs using a pair of interdigitating rollers to strengthen and soften nonwoven webs. The '664 patent also discloses an alternative embodiment wherein the nonwoven fibrous web is laminated to the thermoplastic film prior to intermesh stretching.
Efforts have also been made to make breathable non-woven composite barrier fabrics which are impervious to liquids, but which are permeable to water vapor. U.S. Pat. No. 5,409,761 is an example of a fabrication process from the patent art. According to this '761 patent, a nonwoven composite fabric is made by ultrasonically bonding a microporous thermoplastic film to a layer of nonwoven fibrous thermoplastic material. These methods and other methods of making breathable laminates of nonwoven and thermoplastic materials tend to involve expensive manufacturing techniques and/or expensive raw materials.
Notwithstanding the extensive development of the art for making breathable microporous films and laminates to provide air and moisture vapor permeabilities with liquid-barrier properties, further improvements are needed. In particular, improvements are desired for producing microporous film products and laminates on high-speed production machinery. It would be very desirable to produce microporous film products without undesirable pin holes and without draw resonance. In the past, attempts to increase production speeds have resulted in film breakage or film products with inconsistent properties.